<form action="" method="get">
    <input type="text" name="szukaj" value="<?= htmlentities($szukanyTekst, ENT_COMPAT,'UTF-8') ?>"/>
    <input type="hidden" name="strona" value="<?= $_GET['strona'] ?>"/>
    <input type="submit" name="akcja" value="szukaj" />
        </form>